When you calculated the 30 servings for the 2 tier cake which serving size did you use?
When I try to figure out what size cakes I need to make to accommodate the requested servings I always use the party size not the wedding. This way you are sure there is enough cake no matter how they cut it. That being said if I was to charge per serving I would be use the one larger party size.
You may just have to explain to the customer that if she wants a tiered cake it feeds way more than 15 people. Hopefully they will understand and you have to still do double the work and will not cringe at the price tag.
